The invention relates to a method and apparatus for mixing fine material using a mixing silo to the upper region of which the material to be mixed is delivered and which contains in the center of the silo base a discharge chamber provided with a cover, the material being pneumatically loosened in the region surrounding the discharge chamber and conveyed to the discharge chamber.
A method and apparatus of the aforesaid type is known for example from German Offenlegungsschrift No. 26 57 597. In practice it has often been found that the material located above the cover of the discharge chamber does not take sufficient part in the mixing process because it is not caught by the discharge funnels formed by the pneumatic loosening arrangements. In order to provide a specific remedy for this, it is already known (see German Offenlengungsschrift No. 26 57 597) to provide an opening with a slide valve in the cover of the discharge chamber so that the material located above the discharge chamber can be introduced through this opening directly into the discharge chamber which also serves as a mixing chamber.
This results in a certain improvement in the homogenising effect, but even such a method still has the fundamental disadvantage that so-called long-term fluctuations of the material to be mixed are not equalised. These are fluctuations in the composition of the raw material, which often occur with a frequency of more than 10 to 20 hours and are not equalised because of the significantly shorter period of dwell of the material in the silo. In order to equalise such long-term fluctuations with the conventional mixing method, the mixing silo would have to be of such a size as to involve extraordinarily high construction costs.
Mixing methods and apparatus are also known (see German Pat. Nos. 635,202 and 669,206) in which in mixing silos of a substantially different type (namely without a discharge chamber provided with a cover in the center of the silo base) the material which is pneumatically loosened in the lower region of the silo is conveyed back to the upper region of the silo and is there delivered again together with the fresh material.
Insofar as these known mixing methods and apparatus can be operated continuously, there is a danger that streams of insufficiently mixed material may pass through to the central discharge because of the absence of a discharge chamber provided with a cover. Also, since the lower inlet nozzles of the uptake pipes are located above the ventilated cross-sectional zones of the silo base, it is preferred in the known method for the material which is already fluidised to be conveyed back to the upper region of the silo, whilst the material in less active zones of the silo participates in the circulation. These known methods are therefore not suitable for equalising long-term fluctuations of the material to be mixed.
The object of the invention, therefore, is to provide a mixing method and apparatus of the class described which effectively suppresses or equalises long-term fluctuations of the material to be mixed with low construction and running costs and in particular without any significant increase in the size of the mixing silo.
This object is achieved according to the invention in that mterial is extracted from the zone of the silo located above the cover of the discharge chamber and delivered back to the upper region of the silo in an outer cross-sectional zone which surrounds an inner cross-sectional zone intended for delivery of the fresh material.